Category Archives: Guitar Amps

Two Sound Clips

I’ve been promising these for a while – it was worth the wait. The clips were recorded using my new cabinet – with the 6 inch speaker – at low volume but you can’t tell.

The two sound clips were recorded using an SE2000 mic into a Focusrite Saffire 6 USB audio interface. The mic was 6 inches (15cm) away from the speaker and right on the axis. The rhythm part was panned hard left and the lead part hard right in order that you can easily separate the individual recordings. The recording level was not changed during the creation of these clips and the relative volumes of the individual recordings was not changed. The clips were normalised after being combined into a stereo mix but that is the only change that was made to the recorded signal.


This was played on a Parker NightFly HSS Superstrat at 0.5 Watt. Gain was set for 1-2 o’clock for both the rhythm and lead parts and the tone controls were left top dead-centre. The rhythm part was played on the neck/middle “inbetween” pickup position – you can hear the amp is right on the edge of break-up. Lead was played on the neck, middle and bridge (humbucker) one after the other in that order.


This was played on an all-mahogany PRS single-cut with P90 pickups. They’re hotter than most other pickups so push the amp hard. The amp was set to 0.25 Watt and a little bass was taken off  and a little treble added. Gain for the rhythm part – which was played on the neck pickup – was set at 2-3 o’clock and the lead part – played on the bridge pickup – was on max (of course).

The Origins of the Open-backed Speaker Cabinet

If you’ve been following my posts you’ll know that I’ve spent a lot of time recently researching guitar loudspeakers and their behaviour in different cabinets. What’s most fascinating is that, in contrast to the world of hi-fi loudspeakers, no-one seems at all interested in the acoustic properties of guitar loudspeakers. As David B. Weems remarks in the 3rd edition of Designing, Building and Testing Your Own Speaker System:  “The same laws of acoustics prevail for musical instrument speakers as for stereo speakers, but you wouldn’t know it  by inspecting many commercial musical-instrument speakers”. He goes on to talk about the apparent preference for open-backed cabinets but he obviously isn’t entirely sure why this should be. It’s a good question: Why do guitarists use open-backed cabinets?

There are problems with open-backed cabinets that the hi-fi designers of today would not tolerate but it was not always thus. Referring to The Why and How of Good Reproduction by G. A. Briggs, it is clear that, back in the 1940s and 1950s, the speaker manufacturers knew very little about the physical properties of a loudspeaker that would influence the cabinet proportions; they knew only the resonant frequency, the frequency response and the impedance. The Thiele/Small parameters, that are taken for granted today in hi-fi circles, would not appear until the 1960s/70s so back in the 1940s/50s creating a cabinet that would allow a speaker to do its job properly was trial and error and the cabinet (probably ported) would have been large. This was the world in which Leo Fender built his first amps.

In the mid 1940s, Leo Fender selected a Jensen speaker for his new amp because he liked the tone it produced. It is important to realise that, at that point in history, all speakers were general-purpose speakers. Jensen speakers from the 1940s were made for hi-fi but ended up in guitar amps because – by today’s standards – they were heavy-duty speakers. Jensen stopped making speakers in the 1960s but the relaunched, Jensen-branded speakers are, by all reports, faithful to the originals so we have a pretty good idea what Leo Fender was working with. Leo’s motivation was to make product that was mass-producible and affordable (i.e. cheap) just like a radio set. If a radio set had a back it would be lightweight and vented to allow the valves/tubes inside to stay cool. Removing the back of a radio would reduce resonances and rattles which would make it sound better (we have Gilbert Briggs word for that). Under these circumstances why would Leo Fender want to put a back on his guitar amp? It would make the amp, larger, heavier, harder to cool and would increase his R&D costs. It was a no-brainer to leave the back off.

So open-back guitar amps/cabinets were produced not because they sounded better than the alternatives but because they sounded good enough and were cheaper than the alternatives. I think that was a good design decision at that time but what puzzles me (and David B. Weems)  is why, 60 years later, we are still taking the same approach. That’s a subject for another day.

Guitar Speakers @ Home (Part 3)

underlay1After much experimentation and extensive listening/playing tests, it became obvious that 12mm plywood is a little too flexible for the larger panels of loudspeaker cabinets and bracing is necessary. You can see the diagonal braces that I added in the picture (one per side-panel); their addition was enough to tighten up the bottom-end.

The carpet underlay that you can also see in the photograph was purchased from the Carpet Underlay and has been very effective at cutting out internal reflections and further improving the “punch” of the speaker’s delivery. At a little over £2.00 (ex VAT) per square metre, it’s something of  a bargain.

The sound quality is pretty astonishing; it puts a smile on my face every time I play through it. Right now I can’t get enough of it – I really must get around to posting some new sound samples.

Guitar Speakers @ Home (Part 2)

cabinetThe picture shows a Jensen MOD 6-15 speaker in a 50 Litre enclosure. It’s a big box – around two feet tall – but, because the speaker is designed to work in an open-backed cabinet, it’s smaller than it needs to be for maximum bass response (as calculated using the Thiele/Small parameters provided by Jensen).  However, the bass roll-off is not dramatic and the bass response is restored by pushing the cabinet up against a wall (that’s the proximity effect at work).

So how does it sound? It’s killer – a joy to play through. There is no lumpiness or bass emphasis as there is with an open-backed cabinet (because of the inherent comb filter). In fact, on paper,  the frequency response of this speaker is very similar to the coveted P10R Alnico speaker – a fact that was confirmed by my tests – but it costs a fraction of the price. The only downside is that the MOD 6-15 speaker is 5dB less efficient than its Alnico sibling but that’s not really a drawback when you’re playing at home; 1W has power to spare.

This definitely the way to go for those who want a good sounding setup for home use at spouse- or parent-friendly volumes. I know this sounds unlikely – after all it’s using a cheap and cheesy 6 inch speaker like that in a Champion 600 – but the large cabinet allows the speaker to work to the best of its ability. The rule here is that it’s better to have a cheap speaker in the right cabinet than an expensive speaker in the wrong one.

Guitar Speakers @ Home

Not only is one watt too loud for the home but guitar speakers are not designed for home use. Guitar speakers are largely designed to be stuck into open-backed cabinets which means they are incredibly sensitive to placement. It’s fine if you’re up on stage with a few feet behind the cabinet but at home you’ll want to push your amp out of the way – up against a wall or in a corner – and that will cause the sound coming out of the back of the speaker to bounce around and interfere with the sound coming out of the front of the cabinet. The frequency response of the system – including the room – will be “lumpy” with some frequencies being affected more than others.

The way to fix this is (obviously) to put the speaker in a closed back cabinet but guitar speakers are designed for use in an open-backed cabinet so require a relatively large closed-back cabinet to provide a sensible frequency response. Jensen provide a good range of small speakers that ought to work well in a domestic environment and they publish the Thiele-Small parameters required to design the cabinet. Celestion don’t do either; they have one eight inch driver and that’s as small as they go. Celestion do not appear to publish Thiele-Small parameters.

What does this tell us? It tells us that products aimed at guitarists are remarkably conservative; in many ways they haven’t changed much over the decades. Guitar amplifiers first appeared in the 1930s/1940s which is around 30 years before Thiele and Small published the results of their study into the parameters that were important to the design of a loudspeaker enclosure. Designers back then were working blind so no wonder they left the back off the cabinets; all resonance problems were solved immediately (as long as you kept the cabinet in an unconfined space).

So how do you build a speaker cabinet for a guitar that will work well in a home environment? The answer is not clear to me yet but I want great tone at an acceptable volume and I want to be able to play along with acoustic instruments without dominating the sound so I’m going to have to sort it out. Perhaps the best idea is not to use a guitar loudspeaker at all but this is a bit radical and I do like the sound of Jensen speakers. While I’m pondering this, here are a couple of interesting articles on the subject of guitar loudspeakers:

Note that the Duncan’s Amp Pages article suggests a Qtc should be between 0.707 and 1.0. This is the hi-fi approach but cannot be achieved by any of the traditional Jensen speakers below 12 inches in diameter as the Qts is greater than 1.0 (and the Qtc cannot be less than the Qts) . The  modern Jet series creates some exceptions to this but the smallest Jet speaker is 10″ in diameter which, I would argue, is too large for home use.

As Quiet as a (very noisy) Mouse

littleone-1.4-externalWhile it might seem crazy to add a variable HT/B+ to a One Watt amp, that is exactly what I have done and I am very pleased with the results. You may recall from an earlier post that I found that even a “puny” 1W amp was too loud in some circumstances; this post describes the solution that I chose.

Denis Cornell takes his Romany Plus amp down to 50mW and, while I was skeptical that 50mW would be a useful amount of power, it had to be worth a try. In fact I decided to go for 100mW as it should sound half as loud as 1W. The white, chicken-head knob that you can see in the picture allows the power output to be scaled from 10% to 100%. The black dots around the knob indicate the positions of eighth-, quarter-, half- and full-power. Click here to view the schematic (the new section is around the Power MOSFET Q1).

I only completed the work on it today but I am really impressed. At the lower power settings there is no clean headroom but that’s  the point. You reduce the HT/B+ to get distortion at ultra-low volumes; If you want clean you can increase the HT/B+ and reduce the gain. The tone of the amp doesn’t really seem to change as you change the power output although down at 100mW it loses a little sparkle; I prefer 0.25W.

littleone-1.4-internalI won’t go into the details now but I will be updating the Little One website soon and you can read the final section of Merlin Blencowe’s book on Power Supplies if you need any information about the MOSFET voltage follower.

Little One was only ever intended as a prototype but I’ve been using it for years. It looks tatty and the circuit has grown organically to the point it will be tricky to change as you can see from the picture. It’s about time I rebuilt it.

Update: Having played the modified amp for a couple of weeks now, I think Denis Cornell got it 100% right (hardly a shock). If you’ve got an efficient speaker, you should be able to run your amp at 50mW. This will allow you to get some bite and crunch at the same volume as an acoustic guitar. To do this change R24 to 130K (120K will do).

One Watt Is Too Loud

I was out playing music at a friend’s house and took Little One along to show it to the other guitarists that were there. They loved it, which was reassuring  (it sounds great with a boost pedal in front of it), but what sticks in my mind most is that it wasn’t drowned out by the drummer. Okay, we were playing in a kitchen and the drummer wasn’t hitting the skins as hard as he could but the fact remains; one watt is louder than you might think. In fact, one Watt is half as loud as 10 Watts which is half as loud as 100 Watts so one Watt is pretty damn loud.

It’s easy to miss that something is unacceptably loud when you’re playing on your own; your ears quickly become accustomed to the volume but the neighbours don’t! Guitar amps designed specifically for home use should be as quiet as possible without sacrificing tone. How quiet can the turned-up-to-eleven tone be? That’s what I’ve got to find out.

Off the top of my head there are three ways to attenuate the output:

  1. Attenuate the signal after the output transformer making sure that the amp always sees the required load. The Vox AC4 has an in-built resister network to drop the output from 5W to 1W to 0.25W. Using resistors will change the tone of the out put stage so is not the best appoach but there is a hint that 0.25W is a useful amount of power. Denis Cornell’s Romany amp goes one step further by delivering 0.05W (50mW).
  2. Drop the HT (B+) Voltage using a Zener Diode which will reduce the power of the amp as a whole. To half the power output would require a drop of 75V which is quite a lot but achievable because it’s a low-powered amp.
  3. Drop the HT (B+) Voltage using a VVR circuit which will allow increased flexibility which may be required given what Vox, Cornell and others are doing.

Food for thought.

EQ: The Middle Frequency

During an exchange on tone controls on the AX84 forum I started to wonder why guitars and bass tube amps have such similar tone-stacks. I reasoned that, if the frequency responses of the instruments are different, the frequencies controlled by the tone-stacks should be different.

The middle frequency is the frequency that is left unchanged when you alter the treble and bass controls and is usually around 1kHz. But why?

From what I can see, the reason why it is around 1kHz is lost in the mists of time but it has been suggested that it  is the middle of our hearing range. Listing the octaves in Hertz from the lower to upper thresholds of hearing we have: 20, 40, 80, 160, 320, 640, 1280, 2560, 5120, 10240, 20480. So 640Hz is the middle value which is pretty close to 1kHz (although 1280Hz is closer) so perhaps the suggestion is true. You can go to the Eliot Sound website for a little more information about this but the important question is how does this apply to guitar and bass amps?

I have used the Tone Stack Calculator from Duncans Amps to create PDF files that show the ranges of the standard tone-stacks from Fender, Marshall and Vox. You can see that, although the middle frequencies move around a little, they all agree that it sits in the 500Hz to 1kHz range. It is worth noting that 1kHz is the fundamental frequency that you get when you play the top string of a guitar at the 19th/20th fret so pretty much everything above 1kHz is an harmonic. So the treble control on a guitar amp pretty much controls the amplitude of the harmonics while the bass control controls the amplitude of the fundamentals by raising the output  amplitude of the bottom string relative to the top string (the bottom E-string on a guitar is 82Hz while the top E-string is 330Hz).

This makes some sense for a guitar but a four-string bass guitar generates 311Hz at the 20 fret of its top string. The bottom string is 41Hz when played open and the top string 98Hz. Shouldn’t this be reflected in the tone-stack? The Fender Bassman 5F6-A circuit was copied by Jim Marshall in the early 1960s and, from that point on, there appears to have been little distinction made between guitars and bass guitars until semi-conductors made more sophisticated tone controls affordable.

it may be that properties of the human ear make changes to the tone controls unnecessary. The frequency response of the human ear can be seen on this interesting diagram. Our ears are most sensitive at around 2kHz and our hearing tails off below 200Hz. Some would regard 200Hz as the upper end of “bass” and too much bass between 100Hz and 200Hz can sound “boomy” so it would seem to be sensible to be able to compensate for our poorer hearing and the possibility that the bass might sound boomy by being able to control  these frequencies. That’s almost what the guitar tone-stack does but it starts around an octave higher.

This is interesting stuff and will be the subject of further investigation. I’ll have to leave space on my Northcourt Mk2 layout to try out different tone controls.


After reading Merlin Blencowe’s books (you can find out more about them in an earlier post), I have been inspired to change the circuits of both Little One and Northcourt Mk II. The changes aren’t major but I’m finding Merlin’s books incredibly useful so, if you have any interest in building guitar valve/tube amplifiers buy them before they go out of print (as they surely will).

The NOS Svetlana EF86s arrived from the Ukraine and I love them. I only ordered two but when I heard them I was compelled buy another pair. With one of these in Little One it doesn’t seem to matter what valve/tube you use for the phase inverter or the power stage; it always sounds good. I will produce some recordings ASAP.

I was looking for a can capacitor for Northcourt Mk II and found the AudioCap website. A good source for specialist audio capacitors. The capacitors for the Northcourt rebuild were ordered from there and arrived within a couple of days. Other components will be ordered from AmpMaker but the next step is to plan the layout. More on that later.

The Valve (Tube) Wizard

Designing Tube Preamps for Guitar and Bass

While browsing the Valve Wizard website I discovered that Merlin Blencowe had written and self-published two books called “Designing Tube Preamps for Guitar and Bass” and “Designing Power Supplies for Tube Amplifiers”. I have long been a fan of Merlin’s clear and informative style of presentation so I decided to buy both and I’m glad I did.

Designing Power Supplies for Tube Amplifiers

From my experience it is unusual to find as much useful information in a modern book on Valve/Tube Electronics. I have only had time to read sections of the book about Preamps so far but Kevin O’Connor’s “The Ultimate Tone” is the only other book I have read which has provided so much food for thought. Merlin’s books explain the fundamentals of circuit design so do not make for easy reading but Merlin takes a practical approach which makes the subject easier to digest.

I bought both books from Amazon in the UK. Both books are still available despite Merlin’s warning that the volume on Power Supplies is now out of print. At just over £40 including postage for both books, I think they offer exceptional value for money.